1. Field of the Invention
This invention relates to a combustion system for gas turbine engines or other heat devices, which need to produce very little oxides of nitrogen or carbon monoxide. Specifically, the present invention relates to a system, process, and apparatus for combusting fuel in a gas turbine engine module which dramatically lowers NO.sub.x and CO levels by providing a nearly constant fuel/air ratio in the combustion zone at all engine operating conditions in addition to thoroughly pre-mixing the fuel and air prior to combustion and, if necessary, completely vaporizing a liquid fuel.
2. Description of the Art
Although gas turbine engines do not produce the majority of the nitrogen oxide emissions released into the earth's atmosphere, reducing those emissions will reduce the total and, in that regard, many countries have enacted laws limiting the amounts that may be released. The reaction of nitrogen and oxygen in the air to form nitrogen oxides, like almost all chemical reactions, proceeds faster at higher temperatures. One way to limit the amount of NO.sub.x formed is to limit the temperature of the reaction. The NO.sub.x produced in gas turbine engines is produced in the combustion process where the highest temperature in the cycle normally exists. Therefore, one way to limit the amount of NO.sub.x produced is to limit the combustion temperature.
Various attempts have been made to limit the combustion temperature and thereby NO.sub.x production in both "single stage" combustors (i.e., those having only a single combustion zone where fuel and air are introduced) and "multistage" combustors, including pilot burners where several, serial connected combustion zones having separate fuel and air introduction means are used. U.S. Pat. Nos. 4,994,149, 4,297,842, and 4,255,927 disclose single stage gas turbine combustors where the flow of compressed air to the combustion zone and the dilution zone of an annular combustor are controlled to decrease the concentration of NO.sub.x in the turbine exhaust gases. In the above combustors, essentially unmixed fuel and air are separately admitted to the combustor, with mixing and combustion consequently occurring within the same chamber. See also Japanese Laid-Open No. 55-45739. U.S. Pat. Nos. 5,069,029, 4,898,001, 4,829,764, and 4,766,721 disclose two stage combustors. See also German Gebrauchsmuster, 99215856.0. Again, however, fuel and air are provided to each stage at least partially unmixed with complete mixing occurring within the respective combustion zones.
Attempts also have been made to utilize separate premixer chambers to provide a premixed fuel-air flow to a combustor. Japan Laid-Open Application No. 57-41524 discloses a combustor system which appears to premix only a portion of the total fuel flow to a multistage can-type combustor in a separate mixing chamber prior to introduction to the staged combustion chambers. In U.S. Pat. No. 5,016,443, a large number of separate fuel nozzles is used to inject fuel into an annular premixer chamber. However, the complexity of the above constructions employing multiple fuel nozzles and fuel splitting devices can lead to control difficulties, as well as a high initial cost.